Slim hole drilling

ABSTRACT

A method and apparatus for employing slim hole drilling techniques and avoiding key seat problems, particularly when moving a drill string into or out of the wellbore.

United States Patent Faulk et al.

[15] 3,675,728 [4 1 July 11,1972

[54] SLIM HOLE DRILLING [72] Inventors: Joseph H. Foulk, Dallas; Lhyd R.Kern, Irving; Graham E. Ki, Dallas, all of Tex.

[73] Assignee: Athnflc Rlchfleld Company, New York,

[22] Filed: Sept. 18, I970 [21] Appl.No.: 73,405

[52] U.S.Cl. ..l75/57, 175/325, 308/4A [51] Inl. E211) 17/00 [58] Fieldof Search ..l75/57, 61, 325; 308/4 A References Cited UNITED STATESPATENTS 2,079,449 5/1937 Haldeman ..175/325 Examiner-James A. LeppinkAttorney-Blucher S. Tharp and Roderick W. MacDonald [57] ABSTRACT Amethod and apparatus for employing slim hole drilling techniques andavoiding key seat problems, particularly when a drill string into or outof the wellbore.

12 Chin, 7 Drawing Figures II: ....n... i

V 3 I g M F? INVENTORS Joseph H. Foulk Loyd R. Kern Graham E. King MMATTORNEY PKTE'N'TEDJUL 1 1 1972 SHEET 10F 3 PKTE'N'TEDJUL 1 1 1912 3.675,728

SHEET 2 0F 3 Fig. 3

lo ,6 I i 1 6 so Q I j/ -\Q\ Hg. 7 Fig. 5

INVENTORS Joseph H.Foulk Loyd R. Kern Graham E' King ATTORNEYP'A'TENTEUJULH I972 3,675,728

SHEET 30F 3 Fig. 8

INVENTORS 1 Joseph H. Foulk Loyd R. Kern Graham EhKing BYW ATTORNEYBACKGROUND OF THE INVENTION I-Ieretofore in drilling wellbores drillstrings have been made up of a plurality of sections of drill pipe.Adjacent ends of the drill pipe sections were joined together by adevice normally referred to as a tool joint. A tool joint can be eithera separate coupling means into which is threaded both adjacent ends ofthe two sections of drill pipe, a device where one adjacent end of thedrill pipe has an internally threaded box member into which threads theother adjacent end or pin end of the drill pipe, and the like. Tooljoints normally are of larger diameter than the pipe to which they areattached. In any event, tool joints are the device by which two adjacentsections of drill pipe are joined one to another in a drill string.Therefore, a drill string normally has a substantial number of tooljoints at spaced apart points along the length thereof. At the bottom ofthe drill string is the drill bit and adjacent to the bit in the drillstring there is normally employed at least one drill collar.

The outside diameter of the bit is normally the largest crosssectionallydimensioned member of the drill string because it determines thediameter of the wellbore throufli which all of the rest of the membersof the drill string must pass.

Drill collars are provided so as to impose weight on the bit to improveits drilling rate. These collars are therefore large heavy members whichclosely approach the cross-sectional dimensions of the bit and wellbore.If the drill collar is a square or spirally grooved device its largestcross-sectional dimension will be substantially that of the drill bitand wellbore because with these configurations there are passagewaysbetween the wellbore side and the drill collar which allow movement ofdrilling fluid thereby. If the drill collar is round it will normally beof an outer cross-sectional diameter slightly less than that of thedrill bit or borehole to allow an annulus for the passage of drillingfluid thereby. In any event, the drill collars are large diametermembers which closely approach, if they are not substantially the sameas, the outside diameter of the bit.

The tool joints, on the other hand, being only devices for connectingadjacent sections of pipe, need not be and are not as large as the drillcollars or bit and therefore are of substantially smaller diameter thanthe bit or collars.

The pipe, tool joints, and collars could all be of the same outsidediameter. This situation is quite disadvantageous however in that itnecessitates thicker walled pipe in order to equal the outer diameter ofthe tool joints and collars and this in turn substantially reduces thespace for fluid flow in the annulus between the drill string andwellbore, substantially increases the weight and cost of the pipe, andeliminates the abovementioned-tool joint shoulder that is so useful inconnection with In slim hole drilling very small diameter wellbores,i.e., no

larger than about 6 inches in diameter, are drilled using extremelyhigh, at least about 500 rpm, preferably from about 600 to about 2,000rpm, rotation rates for the drill string. Because of the extremely highrotation rates in the very small diameter wellbore, the drill stringstrikes the side of the wellbore more often during drilling than inconventional rotary drilling. At least two factors that contribute tothe greater frequency of contact between the drill string and the sideof the wellbore are the high rate of rotation of the drill stringcausing lateral vibration of the drill string and the high rate ofrotation of the drill string causing more turns of the drill string perfoot of drilling depth, e.g., 10 to 20 times the rate of rotation ascompared to conventional rotary drilling with three times as fast adrilling rate.

Therefore, in slim hole drilling there is an exceedingly greaterpropensity to the formation of key seats. Key seats are fonned by thedrill string and drill string tool joints bearing against a side of thewellbore and wearing a groove or side hole into the side of thewellbore.

Accordingly, it is very important in successful slim hole drilling toeither avoid the formation of key seats or to have available a device ormethod which renders key seats that may be rendered unobtrusive tosubsequent drilling steps such as removing the drill string from thewellbore to replace a worn out bit and then re-entering the wellborewith the drill string.

In the normal situation where the pipe is of smaller outside diameterthan the tool joint there is provided a shoulder where, in effect, thepipe expands out to the tool joint diarneter. This shoulder is usefulfor conventional smooth wall rig elevators to abut against when liflingthe pipe into or out of the hole.

the smooth wall rig elevators. In this situation slip type elevatorsmust be used on the pipe and with the pipe being heavier than normal andthe like, the slip elevators are more prone to allow the pipe to slidewhile gripped by the elevators than when smooth wall elevators are usedabutting a tool joint shoulder. These elevators have dies which cut intothe pipe weakening it and making it more prone to fatigue type failures.Another system for pulling the flush joint pipe, e.g., to change bits,involves the use of lift plugs or lift nipples threaded into each standof drill pipe to permit engagement of appropriate elevators. This systemis ineflicient because of the excessive time required to install thesedevices when pulling the pipe and to remove them when rerunning the pipeinto the hole to recommence drilling or to perform other operations.

SUMMARY OF THE INVENTION According to this invention, it has been foundthat in slim hole drilling, the problems associated with the formationof key seats can be avoided even though the key seats may be formed byemploying tool joints which have an outside diameter at least as largeas the largest cross-sectional dimension of the drill collar employedand no larger than the outside diameter of the drill bit.

It has further been found that the formation of key seats can be reducedby employing stabilizers and/or pipe protectors along the length of thedrill string so that these elements contact the side of the wellborebefore the tool joints. These elements are provided with abradable outersurfaces so that their surface wears away in preference to the side ofthe wellbore.

Accordingly, an object of this invention is to provide a new andimproved method for avoiding the problems encountered with the formationof key seats. It is another object to provide a new and improvedapparatus which avoids the problems of key seats after they are formed.It is another object to provide a new and improved method and apparatusfor avoiding the formation of key seats during slim hole drilling. It isanother object to provide a new and improved method and apparatus forrendering key seats innocuous.

Other aspects, objects and advantages of this invention will be apparentto those skilled in the art from this disclosure and the appendedclaims.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows a conventional drillstring.

FIG. 2 shows the conventional drill string as it is drilling a wellboreand how key seats are formed thereby.

FIG. 3 shows a cross-sectional view of a keyseat.

FIG. 4 shows how key seats can be a problem upon removal of the drillstring from the wellbore.

FIG. 5 shows a stabilizer means on a drill string.

FIG. 6 shows a cross-sectional view of the stabilizer means of FIG. 5.

FIG. 7 shows a pipe protection means on a drill string.

FIG. 8 shows a drill string with tool joints having an outside diameterat least as large as the drill collars.

More specifically, FIG. 1 shows drill bit 1 with a square drill collar 2adjacent thereto and a round drill collar 3 adjacent to drill collar 2,the bit and collars being supported by a section of drill pipe 4. Pipe 4is joined to an adjacent section of drill pipe 5 by a tool joint 6.Collar 3 is joined to pipe 4 by tool joint 30. Tool joints 6 and 30 areshown as a coupling which is a separate member that is internallythreaded so that for example the adjacent ends of pipe sections 4 and 5can be threaded into opposite ends of coupling 6 thereby mechanicallyjoining pipes 5 and 6.

This sequence of joining adjacent sections of pipe by a tool joint isrepeated for as long a length as desired for the drill string. Normally,there is a tool joint about every 30 feet in a drill string andtherefore with wellbores of depths of 5,000 to 20,000 or more feet therewill be a substantial number of tool joints at spaced apart points alongthe length of the drill string. This is shown in FIG. 2.

Heretofore the relative diameters of the elements were as shown in FIG.1, i.e., dn'll collars 2 and 3 being of about the same diameter as bit 1and tool joints 6 and 30 being of substantially smaller diameter thancollars 2 and 3 and bit 1.

FIG. 2 shows a vertical cross section of a wellbore l drilled in theearth 1 1 with the drill string in the wellbore. It should be noted thatFIG. 2 is not drawn to scale in that the angle of deviation of wellbore10 is exaggerated in order to more clearly show the mechanism by which akey seat is formed and in that the length of drill collars 2 and 3 isnot shown in proper proportion in relation to the drill bit, drill pipesections and the like. However, these variances will be obvious to oneskilled in the art and are done so the better to represent how a keyseat is formed.

Since no'wellbore is exactly vertical and therefore deviates some to oneside of perpendicular as shown in an exaggerated manner in FIG. 2, aportion of the drill string such as that represented by drill pipesections 12, 13 and 14 joined tool joints 15 and 16, will come intocontact with the side of the wellbore during drilling. Since contact ismade during drilling, the drill string being rotated at a very highrate, a side hole 24 will actually be dug into the side of the wellbore.The side hole has a diameter 21 (FIG. 3) of thatof the tool joints'lSand 16. Since the tool joints have a diameter less than that of bit 1side hole 24 will have a diameter less-than that of bit 1 and thewellbore.

This is represented in FIG. 3 which shows the cross-sectional diameterof wellbore as compared to the cross-sectional diameter 21 of side hole24 after being formed by tool joints 15 and 16. Depending upon theconditions of drilling and the particular formations at which the tooljoints 15 and 16 come in contact with the side of the wellbore, the sidehole 24 can have a depth 22which is less than, equal to, or greater thanthe diameter of the drill pipe and/or tool joints.

Referring back to FIG. 2, it is often the case that there are adjacentformations of different strength characteristics pierced by wellbore 10and at which a key seat is formed. The adjacent formations can be, forexample, a coherent formation 17 which maintains the diameter of thewellbore on its own (e.g.,' a sandy formation) and which overlays a lesscoherent formation 18 which does not retain the diameter of the wellbore(e.g., a shale whose clay content is dispersed by the drilling fluid orvibrationally removed by the drill string, and the like). In such asituation there will be formed a cavity 19 in the shale formation 18below the more coherent sand formation 17. This set of circumstancescauses a problem when removing the drill string from the wellbore as isshown in FIG. 4.

FIG. 4 shows that as the drill string is pulled upwardly for removalfrom wellbore 10, drill collar 3, being of a diameter greater than tooljoint 30, will become wedged against formation 17 at about point 31since collar 3 is of greater diameter than tool joint 30 and thereforeof greater diameter than diameter 21 of side hole 24. The drill stringthus becomes hung up in the hole and can not readily be pulled from thewellbore.

According to this invention such problems are avoided in that in a slimhole drilling method comprising rotary drilling a wellbore having adiameter no greater than about 6 inches, preferably from about 2 toabout 5 inches, and rotating the drill pipe and its tool joints at arate of at least about 500 rpm, an additional step is used of employingtools joints which have an outside diameterat least as large as thelargest cross-sectional dimension of drill collars 2 and 3 but no largerthan the outside diameter 20 of bit 1.

The tenn largest cross-sectional dimension" is employed so as to applyto round, square, spirally grooved, or any other configuration of drillcollar or tool joint conventionally used. When the drill collar is roundthe largest cross-sectional dimension is the outside diameter of thedrill collar. When the drill collar is square, the largestcross-sectional dimension is that dimension which extends from one comerof the square to the other corner of the square and will normally besubstantially the same as the outside diameter 20 of bit 1.

The apparatus used according to this invention will comprise a drillstring of at least two sections of drill pipe joined by at least onetool joint, a bit at one end of the string and at least one drill collarin the string adjacent to the bit, the tool joints in the string havingan outside diameter at least as large as the largest cross-sectionaldimension of the collar or collars present but no larger than theoutside diameter of the bit.

By following the teachings of this invention, the largest dimension 21of side hole 24 of the key seat will be at least as large as the largestcross-sectional dimension of collars 2 and 3. Therefore, these collarscan readily be pulled through side hole 24 without hanging up at 31 inFIG. 4.

The largest diameter 21 of side hole 24 can be .of a dimension smallerthan that of the outside diameter 20 of bit 1 because the drill bit isat the end of the drill string and has been found to be sufiicientlyfree to move under the influence of gravity to drag on the lower side 23of wellbore 10 when being removed therefrom thereby avoiding contactwith side hole 24 of the key seat.

The above description relates to that portion of this invention whichreduces the problems encountered when key seats are formed. As an aid inpreventing the formation of key seats in the first place, the drillstring can have at spaced apart points along the length thereof astabilizer means and/or pipe protection means which helps keep the drillstring from contacting the side of the wellbore in the first place andwhich abrades away when it contacts the side of the wellbore so that thestabilizer means or pipe protection means preferentially I wears awayrather than forming a key seat in the wall of the wellbore.

Heretofore stabilizers have been run near the bottom of the drill stringto help keep the wellbore vertical. By this invention stabilizers and/orpipe protectors as hereinafter disclosed are employed at spaced apartpoints along the length of the drill string to help avoid the formationof key seats.

FIG. 5 shows a rotatable stabilizer 40 mounted on a drill pipe section41 below tool joint 42. One or more stabilizers 40 can be mounted oneach or any desired section of drill pipe in a drill string.

As shown in FIG. 6, stabilizer 40 is composed of four spaced apartrubber bearings 50 which bear upon drill pipe 41 and which are supportedby a steel sleeve 51. Around the circumference of sleeve 51 is a rubbermember 52 which has a plurality of rubber projections 53 which extendsubstantially out to the inner wall of wellbore l0.

Bearings 50 can be a single rubber cylinder with grooves therein toadmit lubricating fluid. Nonabradable sleeve 51 must be of a diameterless than the tool joint diameter or else it can become stuck in a keyseat in the same manner as a larger diameter drill collar.

When stabilizer 40 comes in contact with the wellbore elements 53 comein contact with the wellbore wall and member 52, sleeve 51, and bearings50 rotate about drill pipe 41. If any abrading is accomplished in thisencounter the loss is that of elements 53 and not of a portion of thewall of wellbore 10. The spaces 54 between bearings 50 are to allowaccess of drilling fluid to act as lubricant for bearings 50. Bearings50 can be connected through sleeve 51 to element 52 for example byhaving continuity of rubber from bearings 50 through spaced apartapertures (not shown) in sleeve 51 to rubber in element 52.

FIG. 7 shows substantially the same apparatus as that of FIG. 5 exceptthat pipe protection means 60 has been substituted for stabilizer means40. Here again one or more prm tection means 60 can be carried by any oreach section of drill pipe along the length of the drill string.

Protection means 60 is composed of a rubber sleeve having a passageway61 therethrough. Passageway 61 is of a diameter slightly smaller thanthe outer diameter of drill pipe 41 so that the protection means, whenstretched to slide over drill pipe 41, will snugly grip the outside ofpipe 41. The outer diameter of protection means 60 is slightly largerthan the diameter of the tool joints so that when the drill string movestowards a side of the wellbore an outer surface of protection means 60touches the wellbore first and will be abraded away before tool joint 42touches the wellbore side.

Any nonabradable element of stabilizer 40 and protector 60 must have anouter diameter less than that of the tool joints to avoid sticking in akey seat.

Of course, in some circumstances stabilizer 40 and protector 60 may beabraded away to an extent where tool joint 42 can contact the side ofwellbore 10. In this situation it is important that the sizing of thetool joints with respect to the nonabradable parts of 40 and 60, thedrill collars, and the bit be according to this invention as describedhereinbefore. In this manner, even if a side hole is formed afterabrasion away of members 40 and 60, the drill string can still beremoved from the wellbore because the side hole diameter 21 will belarger than the diameter or largest cross-sectional dimension of drillcollars 3 and 2, respectively.

The abradable material on stabilizer 40 and protector 60 can be anymaterial which will be worn away in preference to the side of thewellbore and, although rubber is presently preferred, can be any of alarge number of possible materials including plastics, such aspolypropylene, polypropalene, nylon, teflon, and the like, malleablemetals such as lead or metal alloys, and the like. The abradable portionof stabilizer 40 and protector 60 is also of value in case the drillstring becomes hung up in the wellbore in that it can be sheared offwithout causing the failure of a tool joint or other necessary part ofthe drill string thereby freeing the drill string for movement.

FIG. 8 shows a bit 60 connected to round drill collar 61. The remainderof the drill string is composed of drill pipe sections such as 63 and 65joined by tool joints such as 62 and 64, the tool joints having anoutside diameter at least as large as the outside diameter of drillcollar 61.

EXAMPLE An oil well was drilled in the Bob K Field of Cooke County,Texas to a total depth of 4,180 feet using a drill string composed ofelements substantially as that shown in FIG. 2. The wellbore was drilledwith a diamond bit having an outside diameter of 3 iltinches. The drillstring was composed of drill pipe sections 2 1/16 inches in outsidediameter joined by 2 %inch outside diameter coupling tool joints andhaving a round 2 %inch diameter drill collar mounted above a 3 riiinchoutside diameter spirally grooved drill collar, the spirally grooveddrill collar being adjacent to the diamond bit. The wellbore had adeviation from vertical of 2 maximum.

Upon attempting to pull this drill string from the wellbore startingwith the bit at a depth of 2,847 feet in the wellbore, the drill stringbecame hung up in a key seat as was determined by results while carryingout a fishing operation and later confirmed by running a wirelinecaliper tool. The drill string hung up a plurality of times in a singletrip out of the wellbore.

For comparison purposes, substantially the same drill string wasthereafter employed in the same wellbore except that the drill collarsused had a maximum cross-sectional dimension or diameter of 2 iiiinches.When running this drill string no drill string sticking was encounteredupon its removal from the wellbore starting with the bit at a depth of4, l 80 feet.

Reasonable variations and modifications are possible within the scope ofthis disclosure without departing from the spirit and scope of thisinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A slim hole drilling method comprising rotary drilling a wellborehaving a diameter no greater than about 6 inches, rotating the drillpipe and its tool joints at a rate of at least about 500 rpm, andemploying at least one drill collar adjacent the drill bit, said tooljoints having an outside diameter at least as large as the largestcross-sectional dimension of said at least one drill collar but nolarger than the outside diameter of said drill bit.

2. A method according to claim 1 wherein said wellbore diameter is fromabout 2 to about 5 inches and said rotation rate is from about 600 toabout 2,000 rpm.

3. A method according to claim 1 wherein there is employed a pluralityof rotatable stabilizers at spaced apart points along the length of thedrill pipe and/or drill collars, the stabilizers having at least anexternal portion which is composed of an abradable material and whichhas an outside diameter at least as large as the outside diameter ofsaid tool joints but no greater than the diameter of said bit, allnonabradable parts of said stabilizers being of an outside diameter lessthan the outside diameter of said tool joints.

4. A method according to claim 3 wherein said abradable material isrubber.

5. A method according to claim 1 wherein there is employed a pluralityof pipe protectors at spaced apart points along the length of the drillpipe, said protectors having at least an external portion which iscomposed of an abradable material and has an outside diameter at leastas large as the outside diameter of said tool joints but no greater thanthe diameter of said bit, all nonabradable parts of said stabilizersbeing of an outside diameter less than the outside diameter of said tooljoints.

6. A method according to claim 5 wherein said abradable material isrubber.

7. A slim hole drill string for drilling wellbores having a diameter nogreater than about 6 inches, comprising at least two sections of drillpipe joined by at least one tool joint, a drill bit at one end of saiddrill string, at least one drill collar in said drill string adjacentsaid drill bit, said at least one tool joint having an outside diameterat least as large as the largest cross-sectional dimension of said atleast one drill collar but no larger than the outside diameter of saiddrill bit.

8. An apparatus according to claim 7 wherein there is a plurality ofrotatable stabilizer means at spaced apart points along said drillstring, said stabilizer means having at least an external portion whichis composed of an abradable material and which has an outside diameterat least as large as the outside diameter of said at least one tooljoint but no greater than the diameter of said bit, all nonabradableparts of said protector means being of an outside diameter lesS than theoutside diameter of said at least one tool joint.

9. An apparatus according to claim 8 wherein said abradable material isrubber.

10. An apparatus according to claim 7 wherein there is a plurality ofpipe protector means at spaced apart points along said drill string,said protector means having at least an external portion which iscomposed of an abradable material and has an outside diameter at leastas large as the outside diameter of said at least one tool joint but nogreater than the diameter of said bit, all nonabradable parts of saidprotector means being of an outside diameter less than the outsidediameter of said at least one tool joint.

11. An apparatus according to claim 10 wherein said abradable materialis rubber.

12. An apparatus according to claim 10 wherein there is a plurality ofrotatable stabilizer means and pipe protector means at spaced apartpoints along the length of said drill string.

t t i i

1. A slim hole drilling method comprising rotary drilling a wellborehaving a diameter no greater than about 6 inches, rotating the drillpipe and its tool joints at a rate of at least about 500 rpm, andemploying at least one drill collar adjacent the drill bit, said tooljoints having an outside diameter at least as large as the largestcross-sectional dimension of said at least one drill collar but nolarger than the outside diameter of said drill bit.
 2. A methodaccording to claim 1 wherein said wellbore diameter is from about 2 toabout 5 inches and said rotation rate is from about 600 to about 2,000rpm.
 3. A method according to claim 1 wherein there is employed aplurality of rotatable stabilizers at spaced apart points along thelength of the drill pipe and/or drill collars, the stabilizers having atleast an external portion which is composed of an abradable material andwhich has an outside diameter at least as large as the outside diameterof said tool joints but no greater than the diameter of said bit, allnonabradable parts of said stabilizers being of an outside diameter lessthan the outside diameter of said tool joints.
 4. A method according toclaim 3 wherein said abradable material is rubber.
 5. A method accordingto claim 1 wherein there is employed a plurality of pipe protectors atspaced apart points along the length of the drill pipe, said protectorshaving at least an external portion which is composed of an abradablematerial and has an outside diameter at least as large as the outsidediameter of said tool joints but no greater than the diameter of saidbit, all nonabradable parts of said stabilizers being of an outsidediameter less than the outside diameter of said tool joints.
 6. A methodaccording to claim 5 wherein said abradable material Is rubber.
 7. Aslim hole drill string for drilling wellbores having a diameter nogreater than about 6 inches, comprising at least two sections of drillpipe joined by at least one tool joint, a drill bit at one end of saiddrill string, at least one drill collar in said drill string adjacentsaid drill bit, said at least one tool joint having an outside diameterat least as large as the largest cross-sectional dimension of said atleast one drill collar but no larger than the outside diameter of saiddrill bit.
 8. An apparatus according to claim 7 wherein there is aplurality of rotatable stabilizer means at spaced apart points alongsaid drill string, said stabilizer means having at least an externalportion which is composed of an abradable material and which has anoutside diameter at least as large as the outside diameter of said atleast one tool joint but no greater than the diameter of said bit, allnonabradable parts of said protector means being of an outside diameterless than the outside diameter of said at least one tool joint.
 9. Anapparatus according to claim 8 wherein said abradable material isrubber.
 10. An apparatus according to claim 7 wherein there is aplurality of pipe protector means at spaced apart points along saiddrill string, said protector means having at least an external portionwhich is composed of an abradable material and has an outside diameterat least as large as the outside diameter of said at least one tooljoint but no greater than the diameter of said bit, all nonabradableparts of said protector means being of an outside diameter less than theoutside diameter of said at least one tool joint.
 11. An apparatusaccording to claim 10 wherein said abradable material is rubber.
 12. Anapparatus according to claim 10 wherein there is a plurality ofrotatable stabilizer means and pipe protector means at spaced apartpoints along the length of said drill string.